To make the sauce:

Place the orange juice, mustard, horseradish, mayonnaise and garlic in a small bowl and whisk to combine. Cover and refrigerate until ready to use.

To make the burgers:

Heat the oven to 400 degrees F. Place 2 cups of the orange juice and water in a small saucepan, set over high heat and bring to a boil. Add the rice, cover and reduce the heat to maintain a bare simmer. Cook until the rice is tender, 55 to 60 minutes.</p In the meantime, place the sweet potatoes, chickpeas and remaining 1/2 cup orange juice in a small baking dish. Cover and roast until sweet potatoes are tender, 35 to 40 minutes. Transfer the potatoes and chickpeas to a food processor and process until somewhat, but not completely smooth. Transfer the mixture to a large mixing bowl and add the rice, breadcrumbs, salt, cumin, garlic powder and coriander. Stir to combine. Cover and refrigerate for 1 hour. Shape the mixture into 6 patties. Place 1 tablespoon of grapeseed oil in a large sauté pan, set over medium heat and when the oil shimmers, add the patties. Cook for 4 to 5 minutes or until browned. Add the remaining tablespoon of oil to the pan and carefully flip the patties. Continue cooking until heated through and browned, about 3 minutes. Serve burgers topped with sauce and garnished with arugula and onion.